The present invention relates to liquid crystalline polymers, and in particular to liquid crystalline polymer films used in the formation of multi-layer circuits.
Liquid crystalline polymer (LCP) films are useful in laminates and printed circuits because they typically exhibit low moisture absorption, excellent heat resistance, and good high frequency properties. Despite these advantages, LCP films have not gained wide acceptance and use because they are difficult to laminate consistently. This is due at least in part because the coefficient of thermal expansion (CTE) and the dimensional stability of LCPs are very sensitive to temperature, and the flow properties of LCPs are sensitive to pressure. Additionally, the melt temperature of the LCP film can be altered upon exposure to high temperatures. To achieve uniform lamination of LCP films, lamination equipment must be capable of producing high temperature and pressure conditions in an accurate and uniform manner. Such equipment is costly and has proved to be a barrier to widespread application of LCP film in printed circuits, particularly multi-layer circuits. EP 507 332 discloses how to make a laminate comprising a liquid crystalline polymer using roll lamination, but does not disclose a pretreatment to adjust the coefficient of thermal expansion (CTE).
U.S. Pat. No. 5,360,672 discloses treating a liquid crystalline polymer film below the melt point of the film to improve the cohesive strength of the film, but does not adjust the CTE of the film.
In order to make a multi-layer printed circuit the individual circuits must be bonded together with a material so as to cover the circuit features and act as a dielectric. This can be accomplished by use of a polymeric adhesive or bond ply. Adhesives can be in solid or liquid form. A bond ply is solid and is often preferred because it is non-tacky, self-supporting, and easier to use.
Canadian Application No. 2,273,542 to Forcier suggests use of liquid crystalline polymers as adhesives in the formation of laminates comprising an LCP film bonded to copper, but provides no teaching with respect to the formation of multi-layer circuits using LCP to adhere two dielectric layers, with or without electronic circuitry on their surfaces.
U.S. Pat. No. 6,274,242 to Onodera et al. discloses making a laminate material comprising a liquid crystalline polymer that has been subjected to a heat treatment comprising sequential heating steps below the melt temperature of the film. Onodera et al. thus teaches a specific heat treatment to increase the heat resistance of the film, not the creation of an adhesive layer for circuits.
There are no known reports of the use of LCP films as bond ply layers, most likely because of the above-described sensitivity of the CTE, lamination temperature, and flow properties of LCP films. Currently available LCP films require relatively high lamination temperatures, generally near or above the temperature of conventional electric presses, and are extremely sensitive to temperature uniformity. Accordingly, there is a need in the art for methods for the manufacture of LCP films more suitable for use as bond plies, as well as method for the manufacture of multi-layer printed circuit boards comprising LCP bond plies.
The above-described drawbacks and disadvantages are alleviated by a multi-layer circuit comprising a liquid crystalline polymer bond ply disposed between two circuits, wherein the liquid crystalline polymer bond ply is formed by treating a liquid crystalline polymer film with an amount of heat and pressure effective to produce a liquid crystalline polymer bond ply with an in-plane coefficient of thermal expansion (CTE) of 0 to about 50 ppm/xc2x0 C., and further wherein the multi-layer circuit is formed by lamination at a temperature of 0xc2x0 C. to about 15xc2x0 C. less than the melt temperature of the liquid crystalline polymer bond ply.
Use of the liquid crystalline bond ply in the formation of a multi-layer circuit board allows formation of the multi-layer circuit board under a wider range of temperatures and pressures than heretofore possible. The above discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description.